I have a 4.5.9 install and am running a single email domain (e.g. @domainA). When I try to enter an email address into the "Address to forward mail to" on the Options->Mail tab, if the account name matches an account name in @domainA but the domain is different (e.g. @domainB) I get an "account name@domainB is not a valid email address" message.

When I change the account name to not match one on the system the address is accepted. Any suggestions?